Pick and place machines are generally used to manufacture electronic circuit boards. A blank printed circuit board is usually supplied to the pick and place machine, which then picks individual electronic components from component feeders, and places such components upon the board. The components are held upon the board temporarily by solder paste, or adhesive, until a subsequent step in which the solder paste is melted or the adhesive is fully cured. The individual electronic components must be placed precisely on the circuit board in order to ensure proper electrical contact, thus requiring correct angular orientation and lateral positioning of the component upon the board.
Pick and place machine operation is challenging. In order to drive the cost of the manufactured circuit board down, the machine must operate quickly to maximize the number of components placed per hour. However, as the state-of-the-art of the electronics industry has advanced, the sizes of the components have decreased and the density of interconnections has increased. Accordingly, the acceptable tolerance on component placement has decreased markedly. Actual pick and place machine operation often requires a compromise in speed to achieve an acceptable level of placement accuracy.
One way in which pick and place machine operation is efficiently sped up is in the utilization of a sensor that is able to accurately evaluate both the position and angular orientation of a picked component upon a vacuum nozzle or quill, while the component is in transit to the placement site. Such sensors essentially allow the task of determining the component position and orientation upon the vacuum quill to be performed without any impact on placement machine speed, unlike systems that require separate motion to a fixed alignment sensor. Such sensors are known, and are commercially available from CyberOptics Corporation, of Golden Valley, Minn., under the trade designation Model LNC-60. Several aspects of these sensors are described in U.S. Pat. Nos. 5,278,634; 6,490,048; and 6,583,884.
These laser-based alignment sensors are used in pick and place machines to measure the offset (x, y and θ) and size (Sx, Sy) of picked components. Laser-based alignment sensors generally transmit the measured offset values to the pick and place machine controller, so that the controller of the machine can correct for the offset and accurately place the component upon the circuit board at the placement site. Additionally, the part size (Sx, Sy) features are also measured and transmitted to the pick and place machine allowing the pick and place machine to detect incorrect part size, or other problems.
In a focused imaging system, the relatively large numerical apertures used to acquire images tend to cause dust on intervening surfaces to be appreciably out of focus, which in turn minimizes the effect of dust on the image. However, for reasons of compactness and low cost, it is often desirable to use shadow imaging. Shadow imaging is a technique where illumination is cast upon a component to be detected, and the detector is placed behind the component to detect the shadow cast by the component as the component block some of the illumination. Unfortunately, with shadow imaging, Fresnel diffraction can cause serious disturbances to the image even from a small dust mote.
Providing a shadow-image sensing component sensor for pick and place machines with an improved resistance to disturbances caused by Fresnel diffraction would benefit the art of automated electronics assembly.